Oxidations with Peroxotungsten Complexes: Rates and Mechanism of Stoichiometric Olefin Epoxidations

The stoichiometric epoxidation reactions of a series of olefins with WO-(O2)2HMPT and MoO(O2)2HMPT have been studied in dichloroethane at 40 °C. The data obtained point out the superior oxidizing ability of W(VI) over Mo(VI). However, the two peroxo groups in WO(O2)2HMPT appear to react with the substrates at different rates, the transfer of the first peroxide oxygen being faster than of the second one, contrary to that observed in the behavior of MoO(O2)2HMPT. The crystal structure of WO(O2)2HMPT·H2O, determined by diffractometric techniques, does not show any significant difference compared to that reported for MoO(O2)2HMPT·-H2O, indicating that it might be difficult to correlate the solid state structures with the different behaviors experimentally observed in solution. In spite of the different features shown by the two peroxo complexes, the data collected in this investigation suggest that an identical mechanism of oxygen transfer from the two oxidants is operating. Moreover, this mechanism should not involve the occurrence of substrate-oxidant intermediates.